1. Field Of The Invention
The invention relates to an atomizing device of high melting metal for flameless atom absorption spectroscopy.
2. Description Of Related Art
In flameless atomic absorption spectroscopy, the sample to be analyzed is placed in a cuvette, usually tube-shaped, of graphite, and sometimes of high-melting metals such as tungsten or molybdenum. The sample is heated abruptly by passing electrical current through it. The sample is atomized into an atom cloud in which the elements of the sample to be analyzed are present in an atomic state. This atom cloud is penetrated by an analysis light beam corresponding to the resonance lines of the element to be analyzed. The degree of absorption of the analysis light beam gives an indication of the quantity of the analyzed element. In order to permit precise analysis of the sample, it is necessary to achieve the greatest possible uniformity of cuvette temperature over space and time at the exact moment of atomization in order to avoid matrix effects. Matrix effects can come about because compounds of the analyzed samples or the element to be analyzed with the cuvette material can be stable in zones of lower temperature. This causes a falsification of the analysis result, or at least a reduction in measurement sensitivity.
A design feature which in the past has proven effective to achieve an improved sensitivity of measurement and precision of the analysis is the use of a separate interior carrier element to carry the sample, usually situated inside the cuvette, which is primarily intended to have a certain retarding effect on the heat build-up in the sample area, and thereby an isothermal atomization.
For example, the patents DE-PS 29 24 123 and DE-PS 22 25 421 describe such specialized devices for flameless atomic absorption spectroscopy, which are equipped with a separate interior carrier element.
This special design of an atomizing device has significant advantages with respect to the precision of the analysis. However, the cost of the separate interior carrier element is of about the same order of magnitude as the cost of the cuvette itself and thus raises the total cost of the analysis significantly.
Another feature that has proven effective for achieving a certain retarding effect in atomizing devices in the heating of the cuvette through lateral current lead-ins from the ends.
U.S. Pat. No. 4,407,582, for instance, describes a graphite cuvette where the heating current is exclusively supplied at the ends of the cuvette via Y-shaped contact pieces or a slotted sleeve that makes contact with specially raised sections of the cuvette. With such a design, it is possible to achieve similar properties as with an atomizing device having separate interior carrier elements. However, this design has only proven effective when using graphite as a cuvette material and, due to the specific electrical and thermal properties of graphite, only for cuvettes of especially small dimensions.
The Czechoslovakian patent application 174 728, describes an atomizing device of high-melting metal such as tungsten, molybdenum or tantalum. The device consists of a tubular cuvette with lateral current lead-ins situated parallel with the axis of the tube. It is formed by two strip-shaped sheet metal parts with a semi-cylindrical center portion, gripped in cooled clamping devices at the lateral current lead-ins.
The disadvantage of this type of atomizing device is that it is not possible to achieve a constant temperature distribution over the entire cross-section of the cuvette due to the large heat drain by way of the cooled clamping devices. Moreover, this design requires a separate interior carrier element as well, in order to bring about the desired retardation effect in the heat build-up in the sample area.
The German disclosure document DE-OS 35 34 417 describes an atomizing device of graphite in which the current lead-ins form one unit with the cuvette. In the preferred embodiment, the current lead-ins are equipped with openings in the direction of the cuvette axis which reduce the cross-sectional area. In this manner, a heat drain by way of the cooled clamping devices is largely avoided. But even with this atomizing device it is not possible to achieve a retardation effect in the heat build-up in the sample area. In order to achieve satisfactory precision of the analysis, a separate sample carrier element in the form of a crucible has to be used here as well, which is heated independently of the heating of the cuvette.